Electrical wave filter



Renata/me March 3, 1931.

Inductive L. ESPENSCHIED ELECTRICAL- WAVE FILTER Filed Jan; 3, 1927 2 Sheets-Sheet 1 (awasczltata'ue I N VEN TOR.

BY I k 7 ,V ATTORNEY March 3, 1931. L. ESPENSCHIED 5,

ELECTRICAL WAVE FILTER Fired Jan. 3, 1927 2 Sheets-Sheet 2 a i j PE @5111:

F {3 INVENTOR. ZJK? meme/622206 7 BY A TTORNEY Patented Mar.- 3, 1931 UNITED STATES.

PATENT OFFICE LLOYD ESPENSCHIED, OF HOLLIS, NEW YORK, .ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK ELECTRICAL WAVE FILTER 1 Application filed January 3, 1927. Serial No. 158,753.

This invention relates to electric wave fil ters, and more particularly to a wave filter adapted to transmit with small or neglig ble attenuation currents of all frequencies lying within a range or ranges of predetermined While this invention may find expression.

in a great many embodiments, it has common I to all of these embodimentsthe broad idea of a wave filter involving a plurality of piezoelectric structures, having an impedance element or elements in series with a line and an.

impedance element or elements in shunt across the line, thevalues of these impedance elements being so proportioned that the arrangement will transmit with small or negligible attenuation from a source of electromagnetic energy to some form of electrical receiving or translating device, currents of all frequencies lying ;within predetermined limits or ranges while attenuating and substantially suppressing currents of all frequencies lyin outside'the limits or ranges.

As is well nown in theart, a simple electrical wave filter may consist of an electrical circuit network which has a negligible transmission loss over one or several frequency ranges while the loss is appreciable and may become very high over other ranges. It it is attempted to design an electrical wave filter for high impedance circuits with the elements heretofore employed therein, it will be found that the inductances required become im- "practicably'large and that the capacities be- It is anestablished fact that it is exceedingly diflicult to wind an inductance of one henry or more which is to remain an, inductive reactan'ce over a wide range of frequencies, due particularly to the distributed capacities, of' the inductance.

Furthermore, the electricalresistance of practicable coils tends to reduce the sharpness of the cut-off obtainable in an electrical wave filter and therefore, introduces another serious difliculty. These difliculties'may in part be avoided by the introduction of step-up cuit.

equivalent of a resistance element representand step-down transformers to ad'ust the impedance to the requirements of lter design. Even in such transformers, however,

difliculties arise if the impedance ratio is apparent from the partial equivalence of the piezo-electric crystal to some form of electrical network consisting of suitable reactance elements. That this partial equivalence exists', will readily be seen from the following:

If a piezo-electric. crystal is connected in parallel with a tuned circuit of series inductance and capacity and the arrangement is fed with current from a source of variable frequency, and if, moreover, the current in the tuned circuit may be read on the meter as the frequency of the source. is varied, it will be found that the current reaches a maximum value at the frequency of-resonance of the tuned circuit. If the tuned circuit is soadjusted that its resonant'frequency lies very close to the resonant frequency of natural, vibration of the piezo-electric crystal and if the current in this tuned .circuit is plotted as a function of frequency, it will be found that at the natural vibratory period of the piezoelectric crystal, the current in the tuned circuit will drop to a very low value, rising sharply thereafter as the resonant frequency of the; crystal is passed. In other words, the piezo-electric. crystal acts in some respects like a series resonant circuit of extraordinarily low decrement. at .its resonant frequency. At frequencies somewhat above the frequency at which current through the piezo-,

'electriccrystal is a maximum, the piezoelectric crystal acts like an anti-resonant cir- Moreover, the crystal includes the ing energy dissipation due to its viscosity? A brief consideration of the characteristicsof a piezo-electric crystal aids in a better understanding of the invention.

It is an object of this invention to produce an electrical wave filter which shall include a piezo-electric structure, in order that its transmission characteristic may become flatter within the transmitting hand than has heretofore been possible with other electrical wave filters, so that all frequencies within a definite range or ranges may be transmitted substantially uniformly.

It is another object of this invention to provide an electrical wave filter of a plurality of recurrent sections, each of which in-- cludes substantially equal piezo-electric structures so that its attenuation character istic will be steeper and its transmission characteristic flatter than the corresponding characteristics of electrical wave filters heretofore known.

While the invention will be pointed out with particularity in the appended claims, the invention itself, both as to its further objects and features, will be better understood from the detailed description hereinafter following, when read in connection with the accompanying drawing in which Figure 1 represents a simple reactance curve for a piezo-electric crystal; Figs. 2, 4, 6, 8, 10 and 12 represent recurrent structures "of series and shunt elements of electrical wave filters involving piezo-electric crystals; and Figs. 3, 5, 7 9, 11 and 13 represent the attenuation roperties of the resultant electrical wave ters of this invention. Similar reference characters will be used to designate similar parts throughout the figures ofthe drawing.

Referring to Fig. 1 of the drawing, a characteristic reactance curve for a piezoelectric crystal is shown. The reference character a identifies a characteristic capacitative reactance curve such as might be obtainable when a condenser is connected to; asource of variable frequency, the reactance decreasing as the frequency of a current impressed piezo-electric device.

across the terminals of the condenser increases. The reference character I) identifies a characteristic inductive reactance curve for an inductance connected to a similar source of variable frequency, the reactance increasing as the frequency of the current impressed across the terminals of the inductance increases;

A piezo-electric device may be considered to act like a condenser having a capacitative reactance at all frequenciesv substantially different from the resonant frequencies of the At or neara' resonant frequency, the reactance differs substantially from that of a condenser, its reactance being .then determinable from one of the loops shown in Fig. 1. For the particular piezoelectric device having the characteristic curve shown in Fig. 1, there are two frequencies, or, more accurately, there are two bands offrequencies at which the reactance differs substantially from the normal capacitative reactance of the condenser. These frequencies or hands of frequencies are designated by the reference characters F and F F representing frequencies or a band of frequencies clifferen-t from and higher than the frequencies or band of frequencies F Moreover, at any one of the frequencies Within either band F or F the reactance may be either capacitative or inductive, as will be apparent from an inspection of Fig. 1. Underordinary conditions, the reactance may become inductive at frequencies slightly below those frequencies which correspond to natural frequencies of mechanical vibration of the crystal.

Referring to Figs. 2, 4, 6, 8, 10 and 12 of ,the drawing, each wave filter is composed of identical or substantially indentical sections, respectively, each including suitable elements in series with the line and suitable elements in shunt across the line. Thus, in Fig. 2, two sections of a plurality are shown, each including a piezo-electrie crystal PE in series with the line and a condenser C in shunt across the line. The piezo-electric crystals PE are preferably identical, or substantially identical, in their structure and characteristics and ac-- cordingly vibrate at the same or at substantially the same frequencies. Fig. 3 shows the characteristic reactance curves of the'series and shunt elements of each section, the curve identified by the reference character m cor responding to the series element, the piezo electric crystal PE, and the curve identified by reference character 02 corresponding to the shunt element, the condenser C.

If the piezo-electric crystal PE were connected to a source of alternating current, the frequency of which varied as found desirable, the piezo-electrie crystal would exhibit a capacitative reactance below the frequency f the magnitude of the capacitative reactance decreasing as the frequency f is approached. Beyond the frequency f the reactance would become inductive rather gradually, and as the frequency approaches f.,, the inductive reactance would decrease very suddenly, becoming a capacitative reactance at and beyond the frequency f.,,. The change from a high inductive reactance to a high capacitative reactance takes place about the frequency f and within a very narrow range of the frequency spectrum. Thereafter the capacitative reactance decreases in magnitude with inerease'in frequency.

T he curve Q3 is proportional to the capaci-' apparent from the description of the invention hereinafter following.

It is well known that the condition for unattenuated transmission in an electrical networkexists when the ratio of the magnitude of the series reactance to four times the magnitude of the shunt reactance lies between zero and -1.' Between these limits, the

r transmission takes place, while there is effective attenuation outside of these limits. Currents having frequencies lower than f are considerably attenuated by thestructure of Fig. 2, the attenuation decreasing 'as the fre- Y quency f is approached. At the frequency f the ratio of the series reactance magnitude to the shunt reactance magnitude is zero, so

that transmission-takes place at that fre-.

quency. With frequencies higher than f and lower than f the ratio of the series re-.

actance magnitude to the shunt reactance magnitude is negative with. increasing proportion as the frequency becomes nearer f At thefrequency f however, the ratio of the series to shunt reactance magnitudes reaches the other limiting value for free transmission, i. e., 4, and beyond the frequency f current is eifectivelyattenuated. The range of free transmission lies between the cut-0E points of the filter, f and f Outside of these limits, there is effective attenuation. The structure of Fig. 2 exhibits a flatter transmission characteristic and a steeper attenuation characteristic than have been heretofore obtainable with the ordinary andwell-known electrical wave filters.

There is another very narrow frequency range between the frequency limits f and f which must be considered in connection with the structure of Fig.2. The range of frequencies between f and f satisfy the conditions for free transmission, i. e., the ratio of the series reactance magnitude with the shunt reactance magnitude lies between zero and 4. However, the frequencies within this very narrow range are not freely transmitted by this structure, but are in effect attenuated. Between the frequencies i and f the piezo-electric crystal PE corresponds to an anti-resonant circuit which is receiving current of a frequency corresponding substantially to the natural frequency of the circuit. Accordingly, the resistance of the antiresonant circuit isexceedingly high. This.

exceedingly high resistance effects attenua-. tion in the structure even between the frequencies f and i I Fig. 4 represents another piezo-electric structure having similar series elements, the

piezo-electric crystals PE, and similar shuntelements, the piezo-ele'ctric crystals PE In Fig. 5, reference character-m shows the variation in reactance with frequency in the series elements, the. piezo -electric crystals PE Reference character: "jaz n represents reactance variation with freqnency the shunt ele-' ments, the piezo-electric crystals PE The reference character a shows the filter characteristic of the structure corresponding'to these series and shunt piezo-electric elements. This electrical wave filter freely transmits all currents havin frequencies within the limits f and f and has a very sharply increasing at.-

tenuation characteristic at frequencies outtwo of which are shown. The series element of each section comprises a condenser C and an inductance L in series relationship and the shunt element of each section comprises a piezo-electric crystal PE and an inductance L also in series relationship. The reactance curve for the series elementof each section is designated in Fig. 7 bythe reference character m Thecurve m is proportional to the reactance in the shunt element of .each-section of the filter. This piezo-electric filter transmits a band within the frequency limits f and f and substantially suppresses all frequencies outside. of these limits. At the points 7; and f the ratio of series reactance to shunt reactance equals *4. It is to be noted that the attenuation is infinite at those points of the structure at which the shunt elements have zero reactance. Inasmuch as the shunt reactance curve a. indicates zero reactance attWo points outside of the limits of free transmission of the filter, this filter has infinite attenuation at the frequencies corre sponding to those points.

. Fig. 8 shows another electric wave filter involving piezo-electric crystals. Each series element Includes a piezo electric crystal PE and an inductance L 'in shunt therewith and each shunt element includes an inductance L in shunt with a condenser C. The reactance curves for this piezo-electric filter are shown 1n Flg. 9 of the drawing, in which the reference character :22; designates the series reactance-frequency variation, and in which reference character m represents the shunt reactance-frequency variation, the latter be- "ing obviously'the reactance curveof an antiresonantcircuit. This electrical wave filter has a very flat transmission characteristic be- I tween the frequencies f and f freely and uniformly transmitting currents of all frequencies within these two limits, and it hasa verysteep attenuation characteristic outside of the f and f ,substant1ally suppress currents of 'all frequencies outside of these limits, the attenuation increasing very rapidly with departures from the frequencies f and f Fig. 10 shows another piezoelectric filter having a piezo-electric crystal PE in shunt with an inductance L as the series element of each section and a series resonant circuit including an inductance L and a condenser C as the shunt element of each section. In Fig. 11, the reference character m designates the reactance variation with frequency for the series element, the piezo-electric crystal PE in shunt with the inductance L and the reference character :0 desig- -,nates the reactance variation with frequency for the shunt element, the series resonant circuitcomprising the inductance L and the condenser C. This electrical wave filter is of the high-and-low pass class, freely transmitting currents of frequencies outside of the frequencies f and f while substantially suppressing all frequencies within these limits. At the frequencies f and f the ratio of the series reactance magnitude to the shunt reactance magnitude equals -4, so that currents of these frequencies are freely transmitted by the structure. At frequencies outside of the limits f and f the ratio of the series reactance magnitude to the shunt reactance magnitude lies between zero and 4, the range of free transmission of a filter structure, so that all frequencies outside of the limits f and f are freely transmitted by the arrangement. At frequencies between the limits f and f the ratio of the series reactance magnitude to the shunt reactance magnitude does not lie within the values accepted for free transmission, and accordingly all frequencies within these limits are efiecvtively attenuated.

Fig. 12 represents another piezo-electric filter structure comprising a number of sec tions, each of which has a condenser C as a series element and a iezo-electric crystal PE as a shunt element. urves w, and as show the variations in the reactance magnitude of the condenser C and'of piezo-electrie crystal PE, respectively, with variations in frequency. Ina structure such as Fig. 12, all frequencies between definite limits f and f will be freely transmitted, while all frequencies outside of these limits will be effectively attenuated. At the cutoff points f and f the ratio of the series reactance magnitude to the shunt reactance magnitude again equals 4, so that currents of these frequencies are freely transmitted by the system. At frequencies between these cut-ofi points, the ratio of series to shunt reactance magnitudes lies within the limits accepted for free transmission, and therefore, currents of frequencies within these limits are freely passed by the structure. At the resonant frequency at which the shunt reactance becomes zero, there is infinite attenuation in the arrangement.

By assigning suitable values to the elements employed in the various filter structures shown in the drawing, a system can be provided which will transmit from an input circuit to an output circuit sinusoidal currents having frequencies lying within a preassigned range or ranges, While at the same time effectively suppressing the transmission of currents of other frequencies lying out-- side of the preassigned range or ranges.

It will be understood that suitable impedance elements may be added to the filter structures to bring about any desirable transmission characteristics for these structures, the values of these additional elements being determined from convenience of design or may be made to satisfy some specified requirements, such as, for example, that the system shall have a definite and predetermined impedance at some certain frequency.

, It will be further evident that While the filter structures shown in the drawing are either of the band pass or of the low-andhigh pass classes, any desired high-pass or low-pass filter for a given frequency range or other circuit network may be designed, if the principles of this invention are properly carried out.

It will be obvious that, where a plurality of piezo-electric elements are required, which shall have vibratory periods different from their natural periods, variations may be made in the elements associated with the piezoelectric crystals such, for example, as variations'in the size of the gap existing between one or both of the metallic electrodes and the surfaces of the piezoelectric crystal, within the scope of this invention.

It will also be understood that the num ber of sections required for a particular wave .filter will depend upon the degree to which it is desired to extinguish the currents to be filtered out. If the number of sections is increased, the ratio of the magnitude of the current of any particular frequency entering the filter to the magnitude of the current of that frequency leaving the filter will be greatly increased.

While this invention has been shown in certain particular embodiments merely for. the purpose of illustration, it is to be distinctly understood that the principles of this invention may be applied to other and widely varied organizations without departing from the spirit and nature of the invention and the scope of the appended claims. I

What is claimed is:

1. An electrical wave filter having a sharpenedeut-off, comprising a piezo-electric element, and means co-operating therewith to render its attenuation characteristic very steep near its critical frequency and to render its transmission characteristic very flat throughout a plurality of frequencies.

1 2. An electrical wave filter having recurrent sections, comprising piezo-electric means to give it a steep attenuation characteristic and a flat transmission characteristic.

3. An electrical wave filter having a steep attenuation characteristic and a flat transmission characteristic, comprising a plurality of recurrent sections, each section including substantially equal piezo-electric structures of suitable impedances.

4. in electrical Wave filter having a very sharp attenuation characteristic and a very flat transmission characteristic, comprising a plurality of similar recurrent sections in each of which there is a piezo-electric structure, said piezo-electric structures being substantially similar and of substantially the same impedances.

5. In combination, an input circuit, an output circuit, and an electrical band-pass filter interconnecting said input and output circuits, said electrical bandpass filter including a plurality of sections, each section comprising a reactive net work and a piezo-electric structure, the piezo-electric structure of each section actingto steepen the attenuation characteristic adjacent to the critical frequency of the band-pass filter.

6. In combination, an input circuit, an output circuit, and an electrical network interconnecting said input and said output circuits involving a plurality of piezo-electric elements and a plurality of reactive elements,

one associated with each piezo-electric element, said net work flattening the transmission characteristic throughout a band occupying adefinite portion of the frequency spectrum.

7. An electrical wave filter comprising a plurality of similar recurrent sections, all sectionshaving similar piezo-electric devices of substantially equal vibratory periods and corresponding reactance elements for bringing the series portion of each section into a definite impedance relation to the shunt portion of each section.

8. An electrical wave filter comprising a plurality of sections each having a series element and a shunt element,a plurality ofpiezoelectric structures, and means to co-operate with said piezo-electric structures to bring about a predetermined impedancerelationship between each series element and the I corresponding shunt element.

9. An electrical wave filter comprisin a plurality of recurrent sections, and a p urality o synchronous piezo-e'lectric devices in the limits of the band.

11. An electrical structure capable of freely transmitting currents of all frequencies lying within well defined limits of the frequency spectrum, including a plurality of substan tially similar piezo-electric elements connected in tandem and operating in synchronisin.

12. An electrical band-filter capable of freely transmitting currents of all frequencies lying within well defined limits of the frequency spectrum and for greatly suppressing currents of all other frequencies, including a plurality of piezo-electric devices which have substantially the same vibratory characteristics connected in tandem and means for operating said device so that they may exhibit substantially equal impedances.

13. An electrical wave filter comprising a plurality of-sections including a plurality of substantially equal series piezo-electric ele ments and a plurality of substantially equal shunt piezo-electric elements.

14. An electrical Wave filter including a circuit having a plurality of piezo-electric elements operating in synchronism at one frequency and connected in series relationship,

and a plurality of piezo-electric elements operating inv synchronism. at another frequency and connected in shunt alternately besaid sections, each section having a series element and a shunt element, each series element bearing a definite'and predetermined relation to each shunt element.- p 10. An electricalband-pass filter comprising a plurality of piezo-electric devices connected in tandem, and means whereby all of said piezo-electric devices may be rendered vibratory in response to currents impressed IOU 

